A microporous sheet or film is useful as a printing substrate, such as synthetic paper; as a substitute for leather; as a highly fibrillated sheet which can easily be shredded into fine fibrils to be used as substitutes for paper-making pulps, or as a filter material, such as battery separators.
Many polymeric materials or especially blends thereof are known to undergo fibrillation and/or pore formation upon stretching or drawing. A number of such blends are described in U.S. Pat. Nos. 3,697,367 to Schwarz and 3,511,742 to Rasmussen. Such pore formation may result from different causes, such as separation of phases of incompatible polymer blends, or separation of inorganic polymer fillers like clay or titanium dioxide from the polymer matrix due to stress concentration. Most common in such systems is that the maximum pore formation effect occurs at a draw temperature which is relatively low for the particular polymer system. When the same polymer or blend thereof is stretched at higher temperatures, the pore formation diminishes and a denser film results.
At temperatures where pore formation occurs accompanied by a decrease in density, the draw tension also increases. Draw tension or yield strain also increases with increasing draw rate or operating speed, and reaches the breaking strength of the base film at speeds which are slow and uneconomical for conventional systems used for stretching or drawing of films. Operating a conventional stretching system, such as longitudinal stretching by Goudet rolls and lateral stretching by tenter frames, under tensions which approach the breaking strength of the base film often causes breaks and frequent interruptions of the process. Extrusion speeds are uneconomically slow; for instance, an acceptable draw rate of 200 cm./min. in a single longitudinal draw step over Goudet rolls for a 90 wt. % isotatic polypropylene--10 wt. % polystyrene (See Example 1), would limit the extrusion rate (for a 3' linear die at a draw ratio of 2.0 and a film thickness of 100 micron) to 23.2 lb./hr.
In copending application Ser. No. 614,018, filed Sept. 17, 1976, now U.S. Pat. No. 4,116,892, there is disclosed a method for fibrillating polymer blends of incompatible polymers or filled polymers to form fibrillated or microporous structures by cold drawing at high tension such blends or polymers utilizing the apparatus disclosed in co-pending application Ser. No. 563,623, filed Mar. 31, 1975 now abandoned, both assigned to the same assignee as the present invention.
In the past years, particularly between 1968 and 1973, there have been many attempts to make synthetic paper. Some of such attempts have involved the production of plastic films by extrusion, with the films modified by post-treatment or by additives in the extruded composition to render a paper-like product. Such films could be made suitable for packaging or printing markets which require whiteness, opacity, and a surface suitable for high-quality printing. In an article of Plastic Engineering dated August 1973, there is illustrated the surface effect resulting by admixing a non-crystalline polyethylene with calcium sulfate with a concomitant reduction of density by formation of microvoids of about 40 percent of total volume. Such product, however, was ductile and did not effectively simulate paper formed of cellulose. Products of this type have been developed by many companies in the United States, Japan and Europe. While technical difficulties in producing and using such early synthetic papers have been overcome, such synthetic paper remained too expensive for general acceptance.
Much of the past synthetic paper work has utilized finely divided mineral powders such as clays, calcium carbonate or titanium dioxide to enhance whiteness and/or to make the surface more ink-receptive and more "paper-like". This is a natural extension of known technology of including mineral powders as fillers and extenders in plastic products. The amount of mineral filler in such synthetic papers was generally less than 10% by weight and usually always less than 40% by weight.
The utilization of mineral powders improved the properties of synthetic paper but failed to overcome the uncompetitive cost factor. The mineral fillers failed to reduce the cost per sheet of paper because the cost of the mineral fillers per unit volume in the final product (including compounding cost) is almost the same as the cost per unit volume of the polymer. Therefore, the fillers offered a cost saving per pound of product but did not offer a savings per volume of product, and in the synthetic paper markets the latter is the more important.
There are several patents and publications which describe how mixtures of plastics and mineral fillers can be stretched to yield a porous product of low density. Morris et al (Tappi 45, No. 1: 162) describes the production of porous films by stretching plastic films loaded with mineral fillers treated with organic titanate compounds of about 20 weight percent. Another related development is a laminated film, such as described in British Patent No. 1,268,823 wherein one film of the laminate has a filler content of between 23 to 39 weight percent. Another description of fillers in stretched porous plastics is disclosed by Eller et al (SPE Journal 28, No. 6:54-8), wherein there is produced monoaxially-stretched tapes by stretching a film at 148.degree. C. to form a product having a void volume of less than 40%. However, none of such films have found commercial acceptance.